IEEE 802.11 refers to a set of standards for implementing wireless local area network (WLAN) communication in the, e.g., 2.4, 3.6, and 5 GHz frequency bands. WLAN communication allows a device to exchange data wirelessly with one or more other devices. Wi-Fi Alliance's mark Wi-Fi® is a brand name for WLAN products using any of the IEEE 802.11 standards. IEEE 802.11ac is a new standard being developed to support Very High Throughput (VHT) operations in 5 GHz frequency band. To obtain this VHT operation, an 802.11ac device uses a wide RF (radio frequency) bandwidth up to 160 MHz, up to 8 MIMO (multiple-input multiple-output, which refers to multiple antennas used at both the transmitter and receiver) spatial streams, MU-MIMO (which refers to an multi-user MIMO that allows a terminal to transmit (or receive) signals to/from multiple users in the same frequency band simultaneously), and a high-density modulation up to 256 QAM (quadrature amplitude modulation).
Beamforming is a technique using directional signal transmission or reception with multiple antennas to achieve spatial selectivity. For example, a transmitter can control the phase and amplitude of the signals at each antenna to create a pattern of constructive and destructive interference in the wavefront. To correctly form a beam for MIMO communication, the transmitter needs to know the channel state information (CSI). To obtain the CSI, the transmitter can send a known signal to a device, which allows that device to generate information regarding the channel. The device can then send this CSI back to the transmitter, which in turn can apply the correct phases and amplitudes to form the optimized beam directed at the device. This process is called channel sounding or channel estimation (referenced as the sounding process herein).
In 802.11ac communication, an access point (AP) can use the sounding process to collect CSI from one or more potential destination stations. Thereafter, the AP can use the collected CSI as the current channel estimation to send downlink data to multiple stations in a MU-MIMO frame. Note that the collected CSI can also be used to send downlink data to one station in an SU-MIMO frame, wherein SU-MIMO is a single-user MIMO (a beamforming technique using multiple antennas at one station).